The need for duplex printing from both practical and economic points of view has long been recognized and in classical printing with liquid printing ink, as eg in offset printing of books and journals, duplex printing is common practice.
Electrostatographic printing is based on the image-wise formation of an electrostatic latent image that is developed with electrostatically attractable colorant particles, called toner particles, whereupon the toner image is transferred to the printing stock material, usually paper.
Electrostatographic printing operates according to the principles and embodiments of non-impact printing as described, eg in Principles of Non-Impact Printing by Jerome L. Johnson--Palatino Press--Irvine, Calif., 92715 USA). Electrostatographic printing includes electrographic printing in which an electrostatic charge is deposited image-wise on a dielectric recording member as well as electrophotographic printing in which an overall electrostatically charged photoconductive dielectric recording member is image-wise exposed to conductivity-increasing radiation producing thereby a "direct or reversal mode" toner-developable charge pattern.
By "direct development mode" in electrophotography is meant that toner is electrostatically deposited on the non-photo-exposed areas, whereas in "reversal development mode" toner is electrostatically deposited on the photo-exposed areas. In the last-mentioned development mode a development electrode biased with a charge polarity the same as the polarity of the toner particles ensures that the toner particles are deposited in the photo-exposed areas.
Reversal development mode is not only of interest when negative originals have to be reproduced as positive prints, but likewise when the exposure source is modulated to expose the photoconductor in correspondence with the "black" information to be printed and not in correspondence with the large blank areas of graphic art originals such as printed pages. In that way the exposure source such as a modulated laser source or light-emitting diode array (LED) exposure source controlled normally by a digital electrical signal pattern corresponding with the information to be copied or printed is less loaded.
As used herein, the term "electrostatographic" also includes the direct image-wise application of electrostatic charges on an insulating support, for example by ionography.
Several techniques are known for forming duplex images on a final support medium such as a web or copy sheet. A survey of such techniques is given in U.S. Pat. No. 4,095,979 (Di Francesco et al assigned to Eastman Kodak Company), which relates in particular to duplex copying by means of a photoconductive recording member.
Although most electrophotographic copiers have the capability of reproducing information on both sides of a copy sheet it is not an easy result to accomplish.
In a non-complicated embodiment described in U.S. Pat. No. 3,645,615 (Spear assigned to Xerox Corporation), the copy sheet is redirected into the feed tray of the machine after the first side of the original has been copied to receive a print of the second side of the original on the still blank side. Special paper sheet feed systems have been developed to enable duplex printing at both sides of copy sheets (see for example U.S. Pat. No. 4,095,979 (assigned to Agfa-Gevaert NV).
High volume double side printing (duplex printing) as, eg, in classical offset printing, proceeds on web-type flexible material, normally a roll-fed paper web, which following duplex printing is usually cut into sheets.
In duplex printing on web-type material likewise reversing or turner mechanisms are applied for reversing the web and feeding it into a next printing station [see for example "The Printing Industry" by Victor Strauss, published by Printing Industries of America Inc, 20 Chevy Chase Circle, NW, Washington, D.C. 20015 (1967), p 512-514]. The turnaround of the web to be printed requires an additional roller mechanism and lengthens the part of the printing web residing in the printing machine. Moreover, printing machines operating with web turner mechanisms require more space on the floor of the printing room.
The above cited problems become still more serious the larger the number of printing stations, as is the case in full colour printing operating with three subtractive colour ink printers (yellow, magenta and cyan) and a black printer.
Single-pass colour electrostatographic printers operating with colour printer and black printer stations are described, eg, in U.S. Pat. No. 4,734,788 (Emmett et al assigned to Benson Inc), U.S. Pat. No. 5,027,258 (Tomkins et al assigned to Colorocs Corporation), U.S. Pat. No. 5,160,946 (Hwang assigned to Xerox Corporation) and published PCT patent application WO 92/00645 (Eastman Kodak Company). From these documents can be learned that accurate electrostatographic full colour printing is very complicated.
An example of an electrophotographic duplex printer operating with only two photoconductive rotatable recording drums and single web-type toner receptor material is described in U.S. Pat. No. 3,694,073 (Bhagat assigned to Xerox Corporation). For the exposure of the drums the different sides of an original are illuminated simultaneously and the image-wise modulated light of each side of the original strikes its own photoconductive drum, whereupon the charge image on each drum is toner-developed and the resultant toner images are transferred on opposite sides of the receptor web. According to FIG. 1 of Bhagat, after the first toner image is transferred onto said web the web is moved under a fuser which acts to partially fuse or fix the transferred image upon the web. It has been mentioned that said fusing is optional and preferably incomplete in order that the web be sufficiently cool so as not to adversely affect the transfer of toner to the opposite side. With full fusing the web would have to be quickly cooled before the next toner image is transferred but this requires in practice the lengthening of the path of travel between the fuser and the next corona transfer device.
A problem with non-fused toner on one side of the receptor web passing a next toner-transfer station for attracting a toner image on the other (opposite) side of said web is in that said non-fused toner receives from the corona transfer device a charge opposite to its original triboelectric charge. This will not harm when either in "direct" or "reversal" development mode only two imaging stations with their associated toner-development and toner-transfer stations are used as is the case in the method for duplexing according to said U.S. Pat. No. 3,694,073. However, in multiple colour duplex printing operating with at least three imaging stations in staggered position with respect to the receptor web, an already developed and transferred toner image that has obtained reversed polarity by a transfer corona used for attracting a next toner image to the other side of the web will when coming into close proximity or contact with a next imaging member having a charge opposite in polarity to said toner image become attracted to said member and released from the receptor web whereon it had to stay. However, as the charged toner particles of the first colour on one face of the web reach the oppositely charged drum at the next image-producing station, they are attracted thereto, encouraged by the repulsive force generated by the transfer corona device at that next image-producing station and the already image-wise deposited toner particles are removed from the paper surface. The removal of toner particles in this manner causes a loss of colour density in the final print and a displacement of toner particles may occur at colour boundaries.